Promiscuity may increase the fertility of male offspring, a new study has found. Australian researchers demonstrated that males from polygamous breeds of mice out-compete monogamous males in the race to fertilise females and produce offspring. The researchers also found that mice bred over several generations to have multiple sexual partners produced more sperm and had greater sperm motility than mice bred to be monogamous.

Professor Leigh Simmons and Dr Renée Firman from the University of Western Australia created polygamous and monogamous breeds of mouse, and compared the ability of the male sperm to fertilise female eggs. To create the polygamous breed, female mice mated sequentially with three males, and this was repeated up to 12 generations. For the monogamous breed, females were paired with a single male for each litter.

After 12 generations of breeding, they tested the ability of the males to fertilise females and produce offspring. Female mice were mated with two males consecutively: 16 were mated with a monogamous male first, followed by a polygamous male; and 16 were mated in the reverse order. The researchers then analysed genetic markers of the embryos of the pregnant mice to determine the paternity of the offspring.

They found that polygamous males produced more offspring than monogamous males: they produced 76 percent (%) of the offspring when they were the first to mate with the female, and 58% when they were the last. Polygamous males exclusively fathered 33% of the litters, compared to only 14% of the monogamous males. Fifty-three percent of the litters were of mixed paternity.

Professor Simmons and Dr Firman had shown in an earlier study that, after eight generations, males from the polygamous breed produced more sperm, and sperm with greater motility, than males from the monogamous breed. Female fertility between monogamous and polygamous breed was no different after 8 and 12 generations: females released the same number of eggs per cycle and on average produced the same number of offspring per litter.

When females have multiple sexual partners, sperm from different males compete to fertilise her eggs. The researchers suggest that the selective pressure of sperm competition in polygamous breeds leads males to evolve, over several generations, greater sperm quality and a greater ability to fertilise females compared to monogamous breeds.

'These mice only had a few generations to adapt, perhaps in the following generations we will begin to see larger sperm', said Professor Simmons.

Dr Tommaso Pizzari from the University of Oxford, commented on the study: 'I think (the new results are) an important confirmation of the evolutionary power of polyandry and the sperm competition selection that arises from polyandry'.

The study was published in BioMed Central's journal BMC Evolutionary Biology.

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